Add a .gitattributes file

[jrfnl]

This PR adds a .gitattributes file to keep the archives GH creates of the repo clean of development related files. People using Composer can still get these files in their setup if they really want to, by using --prefer-source.

Refs:

• Reddit: I don't need your tests in my production
• Blog: I don't need your tests in my production

Open questions:

I was not sure whether the following files should be excluded or not. Advise welcome:

• package.json.lock
• renovate.json
• yarn.lock